1. Field of the Invention
The invention is directed to a process for making high-precision, fine-detail forgings, utilizing a workpiece material with superplastic forming characteristics. The process involves high strain rate forming to impart the bulk of the total deformation required and superplastic forming to impact detail and approach final tolerances. The process is suitable for high production rates and minimizes or eliminates secondary machining.
2. Description of the Prior Art
Imparting detail and achieving precision in a forging is a difficult task regardless of the material. In commercial practice, it is common to use several dies to progressively impart detail in the forged component. In such progressive die forging processes, five or more intermediate steps each requiring special tooling may be employed, and still up to 80% of the original workpiece may have to be machined off to achieve final dimensions. Brass is an excellent forging material and many components are forged in a single step when brass is utilized. However, even with brass, high pressures developed in the forging operation limit detail that can be imparted, and again, extensive machining is usually required to achieve final dimensions, particularly for larger forgings, i.e., those weighing 1 pound or greater. Multitooling requirements as well as required processing contribute greatly to forging being considered a relatively expensive process. Many components which could benefit from the strength and toughness of a forging are cast or formed from powders for economical reasons.
Precision forging techniques have been developed which minimize secondary machining for aluminum and titanium, but these techniques are very slow and costly and thus far have been limited in use primarily to the aircraft and aerospace industries.
Work with eutectoid zinc-aluminum alloys has shown that superplasticity can be utilized to forge complex components close to final dimensions. However, superplasticity is a strain rate dependent phenomenon usually effective at relatively low strain rates. This condition leads to slow, expensive processing which has tended to limit commercial utilization.
Recent work with the eutectoid zinc-aluminum alloy demonstrates that this superplastic material behaves in a conventional manner when deformed at high strain rates. Thus, at strain rates typical of a mechanical press, the zinc alloy is not superplastic but displays a flow stress and ductility similar in magnitude to those of common aluminum forging alloys.